See here.

If using wait() doesn't fix it, the only option I can think of at this moment is if you were using a 8Mhz board but compiled the program for a 16MHz...

// Enable and select radio type attached
#define MY_RADIO_NRF24
//#define MY_RADIO_RFM69

#include <MySensors.h>
#include <SPI.h>
#include <dht.h>

#define CHILD_ID_HUM 14
#define CHILD_ID_TEMP 15
#define HUMIDITY_SENSOR_DIGITAL_PIN 5
unsigned long SLEEP_TIME = 900000; // Sleep time between reads (in milliseconds)
int BATTERY_SENSE_PIN = A0; // select the input pin for the battery sense point
int oldBatteryPcnt = 0;

dht DHT;
float lastTemp;
float lastHum;
boolean metric = true;
MyMessage msgHum(CHILD_ID_HUM, V_HUM);
MyMessage msgTemp(CHILD_ID_TEMP, V_TEMP);

void presentation()
{
// Send the sketch version information to the gateway
sendSketchInfo("TemperatureAndHumidity DHT21", "1.1");

// Register all sensors to gw (they will be created as child devices)
present(CHILD_ID_HUM, S_HUM);
present(CHILD_ID_TEMP, S_TEMP);

metric = getControllerConfig().isMetric;
}

void setup()
{
Serial.println("DHT TEST PROGRAM ");
Serial.print("LIBRARY VERSION: ");
Serial.println(DHT_LIB_VERSION);

// use the 1.1 V internal reference

#if defined(AVR_ATmega2560)
analogReference(INTERNAL1V1);
#else
analogReference(INTERNAL);
#endif

}

void loop()
{
//-------------------- battery

// get the battery Voltage
int sensorValue = analogRead(BATTERY_SENSE_PIN);

#ifdef MY_DEBUG
Serial.println(sensorValue);
#endif

int batteryPcnt = sensorValue / 10;

#ifdef MY_DEBUG
float batteryV = sensorValue * 0.003363075;
Serial.print("Battery Voltage: ");
Serial.print(batteryV);
Serial.println(" V");

Serial.print("Battery percent: ");
Serial.print(batteryPcnt);
Serial.println(" %");

#endif

if (oldBatteryPcnt != batteryPcnt) {
    // Power up radio after sleep
    sendBatteryLevel(batteryPcnt);
    oldBatteryPcnt = batteryPcnt;
}

//-------------------- DHT

// READ DATA
Serial.print("DHT21, \t");
int chk = DHT.read21(HUMIDITY_SENSOR_DIGITAL_PIN);
switch (chk)
{
case DHTLIB_OK:
    Serial.print("OK,\t");
    break;
case DHTLIB_ERROR_CHECKSUM:
    Serial.print("Checksum error,\t");
    break;
case DHTLIB_ERROR_CONNECT:
    Serial.print("Connect error,\t");
    break;
case DHTLIB_ERROR_ACK_L:
    Serial.print("Ack Low error,\t");
    break;
case DHTLIB_ERROR_ACK_H:
    Serial.print("Ack High error,\t");
    break;
default:
    Serial.print("Unknown error,\t");
    break;
}
// DISPLAY DATA
Serial.print(DHT.humidity, 1);
Serial.print(",\t");
Serial.println(DHT.temperature, 1);

send(msgTemp.set(DHT.temperature, 1));
send(msgHum.set(DHT.humidity, 1));

delay(SLEEP_TIME);
}

I'm not sure I understand the part "Domonicz receives it by 30min". Regardless of what is finally stored, you should still see the values in the devices displayed correctly when a new data point arrives.